1. Field
The present invention relates to the field of plasma sources for fabrication, pollution abatement, and cleaning processes. More specifically, the present invention relates to exciting the plasma of an inductively-coupled plasma source using pulsed power.
2. Description of the Related Art
Plasmas are typically generated in specially built chambers. Such plasma chambers can be used for gas dissociation in etching, implantation, deposition, or cleaning processes for semiconductor and micro-machining fabrication chambers. The plasma can also be used for materials processing, production of activated gases, pollutant abatement and other applications. In some applications, the plasma is generated from oxygen, argon and fluorinated gases by driving a continuous RF (Radio Frequency) current through the plasma. In such devices, the plasma is electromagnetically coupled to an excitation coil through either the air or a magnetic core. Since the excitation coil and the plasma can be modeled as the primary and secondary windings of a transformer, respectively, these inductively coupled plasma sources are sometimes also called transformer-coupled plasma sources. The plasma chamber may take different shapes, such as cylindrical, toroidal, and others. For semiconductor applications, the transformer's primary winding or coil is typically driven at RF frequencies in the range of 0.2–2 MHz, at power levels in the range of 2–10 KW.
A RF power generator is commonly used to drive the primary coil. The RF excitation for inductively coupled plasmas and other plasma sources is commonly applied in a continuous wave (CW) mode, i.e. the RF power is applied continuously to the plasma. Part of the CW mode delivered power is used to drive the reaction rate inside the plasma chamber, i.e. to dissociate and ionize the feeding gas. The reactions include dissociation, excitation and ionization. The rest of the CW mode power is dissipated from the plasma and deposited on the chamber walls in the form of heat. The dissipated power must constantly be removed to keep the walls of the chamber within the chamber's safe operational temperature range. For effective heat removal, the plasma chamber walls are typically made of thermally conductive metals, such as aluminum, and are cooled by water.
Manufacturing and industrial processes can be improved with yet higher gas flows, gas dissociation and pressures. Increasing the power supplied to the plasma increases the reaction rate within the plasma chamber, allowing the gas flow rate and pressure to be increased. However, higher power levels also increase the power dissipated to the chamber walls. As a result, in plasma sources operated in CW mode, the heat dissipation capability of the plasma chamber walls limits the amount of power that can be coupled to the plasma.